Components of a catalyst are often supplied to a support by contacting the support with a solution containing the component, such as for instance contacting a silica support with a solution of chromium acetate in hexane. See, for example, U.S. Pat. No. 5,895,770.
It is well known in the art to activate a supported catalyst comprising chromium, useful in the manufacture of polyolefins such as polyethylene, by a treatment comprising calcining the supported catalyst in the presence of an excess of molecular oxygen at a temperature in the range of about 300° C. to about 1000° C. for up to about 50 hours. See, for example, U.S. Pat. No. 4,981,831.
U.S. Pat. No. 5,093,300 discloses that after treatment in an oxidizing atmosphere up to about 850° C. a chromium catalyst is cooled down and then treated in a non-oxidizing atmosphere.
U.S. Pat. No. 6,150,572 discloses regenerating a chromium catalyst containing organic contaminants by treatment with an oxidizing gas such as air at temperatures of from 350° C. to 400° C. until the organic contaminants disappear, followed by treatment with hydrogen mixed with an inert gas.
U.S. Pat. No. 6,201,077 B1 discloses a chromium on silica catalyst activated in an oxidizing ambient at from 600-1100° F. (about 315° C. to about 590° C.) useful in producing a polyethylene having high Environmental Stress Crack Resistance (“ESCR”) for blow molding applications. The activation treatment period is from 1 minute to 50 hours. The most preferred activation temperature is from about 900° F. to about 1050° F. (about 480° C. to about 565° C.). U.S. Pat. No. 6,204,346 and U.S. Patent Application Nos. 2001/0004663 and 2001/0007894 disclose similar procedures.
U.S. Pat. No. 6,214,947 discloses treating a chromium catalyst in a dry inert gas, then titanating the catalyst, and then activating with oxygen.
U.S. Pat. No. 6,359,085 (see also the related EP 1038 886 A1) discloses a thermal treatment for a chromium-based catalyst comprising treatment under N2 at 350-850° C. and then treatment under air at 350-850° C. The treatment with both nitrogen and air preferably occurs at or above 480° C., but according to the disclosure the treatment in nitrogen and air must not be carried out at the same temperature and it is preferred that the treatment in air occur at a temperature lower than the treatment under N2. After the treatment with air, the chromium-based catalyst is cooled down to room temperature while replacing the air with nitrogen before contact with ethylene in a polymerization process.
EP 0 882 740 A1 and EP 0 882 743 A1 disclose a supported chromium-based catalyst titanated under specific conditions and used for the homopolymerization or copolymerization of ethylene. After drying in an inert gas at a temperature of at least 300° C., the catalyst is titanated and then activated at a temperature of at least 500° C.
The present inventors have observed that catalysts comprising a support such as silica which is coated with chromium, titanium, and optionally at least one of zirconium, aluminum, and boron, and containing organic residues from the coating process, produce a “heat kick” or temperature spike or exotherm during normal air activation, such as by heated fluidization. While not wishing to be bound by theory, this heat kick is believed to be the result of uncontrolled oxidation of the organic groups. The present inventors have discovered that controlling the temperature spike in the appropriate manner improves the catalyst performance, as hereinafter described.
Embodiments of the present invention may have the advantage over previously known methods of activating supported chromium/titanium catalysts in having one or more of the following: an improved catalyst activity, improved melt index (“MI”) response, an improved ESCR in a polyethylene manufactured using the catalyst prepared according to the present invention, or a combination of these improvements. The catalyst prepared according to the invention may be used to produce polyolefins by solution polymerization, slurry polymerization, and gas-phase polymerization techniques.
Large diameter plastic pipe such as highway drainage pipe is typically made in a continuous extrusion process comprising extruding resin through a die to provide a large diameter tube capable of carrying a fluid. One typical use is as highway and/or storm water drainage pipe. The term “pipe extrusion resin” in the art is used to distinguish this type of hollow tube from conduit resin designed to carry utilities such as wire, cable, and the like. These different uses have different requirements.
The emphasis in the extruded pipe market is for a resin that exhibits high ESCR, that may be easily extruded through a relatively large diameter die, and that also has the appropriate strength characteristics to maintain its integrity during use, e.g., as buried drainage pipe.
High molecular weight, high density polyethylene (HMW HDPE) is used to manufacture storage containers, such as large industrial drums (e.g., 30- and 55-gallons) and intermediate bulk containers (“IBC”) such as 100- and 300-gallon containers. HMW HDPE is also used in sheet extrusion/thermoforming operations to produce large parts such as truck bed liners, “port-a-potties” or portable toilets, and “dunnage trays” for holding and transporting large industrial parts such as transmissions. The end user expects—and governmental regulations often require—that the container meet certain minimum requirements, such as for impact resistance, top load, ESCR, and chemical resistance. In addition, the manufacturer of the containers expect ease of processability. Depending on the end use, there may be even more specific requirements of the material. For instance, in the case of large drums manufactured by blow molding, a high melt strength is generally desired, as the parison produced in the blow molding process typically must maintain its integrity for longer periods of time as the object made gets larger.
High molecular weight HDPE is also used to manufacture HIC containers, and once again, the end user expects certain minimum requirements will be met, as given above.
In the development of resin there is typically a trade off between characteristics such as resistance to slow crack growth and rupture (measured, for instance, by ESCR), stiffness (measured, for instance, by density) and processability or more specifically ease of extrusion (measured, for instance, by MI). Typically the higher the molecular weight of polyethylene, the higher the resistance to crack growth. However, increasing the molecular weight will decrease processability and make extrusion more difficult.
Conduit resin comprising polyethylene resin is typically used in a continuous extrusion process comprising extruding the resin through a die to provide a hollow pipe or conduit which can carry utilities such as wire, cable, fiber optics, and the like. The electrical conduit market alone uses over 300 million pounds of HDPE annually. The term “conduit” in the art is used to distinguish this type of hollow tube from large diameter pipe, such as highway drainage pipe. These different uses have radically different requirements. The emphasis in the conduit market is for a resin that exhibits high ESCR and that may be easily extruded through a relatively small diameter die.
The manufacturers of the conduit typically have an investment in having their extrusion apparatus set to accept a resin having a certain processability range and the challenge for the resin manufacturer is to provide the target processing characteristics while at the same time optimizing end use characteristics as much as possible. The problem is then to supply the appropriate resin with consistent quality and acceptable price.
U.S. Pat. No. 6,403,181 B1 relates to a premium performance polyethylene produced using a metallocene transition metal catalyst, providing a high molecular weight component and a low molecular weight component.
A number of patents are directed to producing HDPE having good resistance to stress cracking, for instance U.S. Pat. No. 6,214,947, WO 00/14129, and EP 0905148. Typically such patents are directed to the catalyst systems employed in the production of the HDPE and more specifically to complicated preparation and/or treatment techniques such catalysts to optimize activity and catalyst life, among other characteristics.
However, what is needed is a process for producing a resin targeted for the pipe extrusion and utility conduit markets, wherein the process uses a readily available catalyst, for instance a commercial catalyst, that may be easily and reproducibility activated and wherein the resultant activated catalyst has high activity and long life.
The present inventors have discovered a method of making pipe extrusion and utility conduit resins having a high ESCR and good processability using a chromium and titanium-based supported catalyst which is commercially available and which may be readily activated for polymerization so as to provide for an excellent MI response, high activity, and long catalyst life.
Embodiments of the present invention may have the advantage over previously known methods of producing conduit HDPE by having improved MI and an improved ESCR.
Chromium catalysts are well known catalysts for olefin polymerization and are useful in preparing HMW HDPE. In these catalysts, a chromium compound, such as chromium oxide, is supported on a support of one or more inorganic oxides such as silica, alumina, zirconia or thoria, and activated by heating in a non-reducing atmosphere. U.S. Pat. No. 2,825,721 describes chromium catalysts and methods of making the catalysts. It is also known to increase polymer melt index by using a silica-titania support as disclosed, for example, in U.S. Pat. No. 3,887,494. Numerous activation procedures have been described in the prior art for optimizing catalyst performance and resultant ethylene polymer characteristics, such as U.S. Pat. Nos. 4,981,831; 5,093,300; 5,895,770; 6,150,572; 6,201,077; 6,204,346; 6,214,947; 6,359,085; and 6,569,960; U.S. Patent Application Nos. 2001/0004663 and 2001/0007894; EP 1038 886 A1; EP 0 882 740 A1; EP 0 882 743 A1; EP 0905148; and WO 00/14129.
What is needed is a resin in particular having a high ESCR, good stiffness, and excellent processing characteristics, produced by a process that preferably can employ a commercially available catalyst, and wherein the activated catalyst has high activity and long life.
The present inventors have discovered a method of making a resin particularly suitable for utility conduit; extruded pipe; household/industrial containers; and large part blow molding applications, particularly drums, IBCs, and sheet extrusion parts, having high ESCR, high impact resistance, high stiffness, and good processability, using a chromium and titanium-based supported catalyst activated in a simple manner so as to provide for high activity, and long catalyst life, said catalyst being commercially available.
Embodiments of the present invention may have the advantage over previously known methods of producing blow molding HDPE by having an improved ESCR versus density relationship, yet maintaining good processing and stiffness characteristics.